Particle-beam apparatus of this kind are used in semiconductor production and microstructuring applications. In particular direct patterning by ion-beam irradiation is a promising concept for the future industrial fabrication of nano-scale devices with high resolutions, in particular the 32 nm and 22 nm nodes and below. The pattern definition (PD) device may be, for instance, a stencil mask or, preferably, a programmable multi-aperture device. The implementation of a multi-beam projection optical system based on a programmable multi-aperture plate allows a significant improvement of the achievable productivity in comparison with focused single beam systems. The reasons for the improved productivity are, firstly, the parallelism of the process using a plurality of beams and, secondly, the increased current which can be imaged to a substrate at the same resolution. Both are made possible by a significantly reduced Coulomb interaction in the beam. Furthermore, the moderate current density related to the projection optical system results in an enhanced process rate when precursor gases are used for beam-induced chemical processes. As compared with a focused beam system, also the beam intensity can be reduced considerably, thus an overly heating effect can be avoided.
When using ions as projectiles in a charged-particle optical system there is always the problem of beam impurities of the ion beams produced by conventional plasma ion sources such as a multicusp plasma source. Such beam impurities are of different mass and consequently different momentum and/or kinetic energy, typically in the range of few percent of the total beam, and will lead to image defects such as tails, double patterns or blurred patterns.
Mass impurities (i.e., impurities having same energy but different mass from the nominal beam ion species) may originate from, for example, residual gas in the multicusp ion source. Mass impurities will cause image defects in combination with unavoidable magnetic fields (typically in the range of 10 nT) which can not be shielded out sufficiently if varying external fields are present.
Particles with deviating energy are, for instance, particles ionized at a different potential than the nominal beam species. In the electro-optical arrangements used to project the particle beam, such energy-deviant particles will experience altered focal lengths and can, in consequence, cause underground dose or radial tails.
Well-known from prior art state of the art is the use of an E×B filter, a so-called Wien filter, or similar arrangements using a combination of electric and magnetic fields. In a Wien-type filter, a particle beam, preferably a beam of small diameter, is exposed to superposed electric and magnetic fields oriented in a manner that only particles of a defined beam energy and mass maintain their initial directions. The other particles are deflected out of the path and filtered out, usually by absorbing the deflected beam portion at a beam limiting aperture or slit diaphragm.
Disadvantages of the state-of-the-art Wien Filters are (i) the Coulomb interaction of the beam particles within the filter, causing an increased aberration and energy spreading in the subsequent electrooptical arrangements, and (ii) the destructive effect of the beam on the apertures (or slit plate) needed for absorbing unwanted beam portions, since it is usually unavoidable that rather high current densities occur. Moreover, the provision of a Wien-type filter increases the length of the optical column and involves additional mechanical and electrical complexity of the setup.
Apart from blurring of the image there is always the problem that ion irradiation may induce damage of beam-limiting elements of the optics, such as diaphragms or beam-shaping elements, where portions of the beam give rise to sputtering or undesired ion implantation. This problem is particularly problematic in connection with beam impurities, which have to be prevented by appropriate means, such as absorbing elements, from reaching the substrate.